This is the 10289th most frequent Japanese word.
頑固
Stubborn; obstinate.
In this sentence, "頑固" (stubborn) is used to describe a person's personality, indicating their unwillingness to change their viewpoint or opinion despite reason or persuasion.
彼はとても頑固で、自分の意見を絶対に曲げようとしない。
He is very stubborn and will absolutely not bend his opinions.
Here, "頑固" is metaphorically applied to describe rust as being stubborn or difficult to remove, showcasing the word's usage beyond describing personal traits.
この古い錠を開けるにはかなり頑固な錆を取り除かなければならない。
To open this old lock, one must remove quite stubborn rust.
In this instance, "頑固" is used to characterize a trait that, while often negative, positively impacted an outcome by showing persistence or determination.